You were 10 seconds faster... ;) There is no need to provide full path. In default configuration apt will look for sourcelist file (and all other important files) in /etc/apt/. @Tshepang: take a look at /usr/share/doc/apt/examples/configure-index.gz - there is an index of all apt configuration options.
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pbmJun 12 '12 at 20:42

This may not quite be what you're looking for, but Debian (and probably Ubuntu by now) supports the /etc/apt/sources.list.d/ directory. You can place files there (so long as they end with "sources.list" (e.g., foo.sources.list)) that will be included by apt when used. Going one step further, you can put them anywhere and symlink from /etc/apt/sources.list.d to the real file.

But as for /etc/apt/sources.list the sources.list(5) man page talks about it as though it's unchangeable.